1. Field of the Invention
The invention relates to an oriented, multilayer polyolefin film comprising a base layer essentially comprising polypropylene, and at least one heat-sealable outer layer. These films are distinguished by a low coefficient of friction in combination with excellent heat-sealability and outstanding antistatic properties.
2. Description of the Related Art
Films having low coefficients of friction are known. The demands on the processing properties of the films and their smooth running through automatic machines have continually increased over the years. For this reason, ever-lower coefficients of friction are required. Today the adjective "low" describes friction values which cover an order of magnitude of from 0.3 to 0.1, whereas a few years ago a friction of 0.4 to 0.5 was regarded as extremely "low".
DE-A-20 01 032 describes films made from various thermoplastics whose surface-slip characteristics have been improved by addition of carboxamides and antiblocking agents. Since it is not possible for a sufficient amount of lubricant to be incorporated into the outer layers alone, the additional incorporation of the amides into the base layer is recommended. These films have a coefficient of friction in the range from 0.4 to 0.8 and thus no longer meet today's quality requirements.
U.S. Pat. No. 4,117,193 describes multilayer films comprising a polypropylene base layer containing a lubricant, an antiblocking agent and an antistatic. The outer layers of these films comprise a polymer blend and additionally contain a lubricant and an antiblocking agent. The polymer blend comprises an ethylene-butylene copolymer and a polyolefinic resin such as HDPE or LDPE. It is disclosed that the poor surface-slip characteristics of the films cannot be sufficiently improved by the addition of lubricants and antiblocking agents alone. For this reason, the outer layer is modified by addition of HDPE or LDPE in combination with a lubricant and antiblocking agent. According to the examples and comparative examples, the reduction in the coefficient of friction is essentially due to the addition of HDPE. Pure copolymeric outer layers with the same additive composition have coefficients of friction of from 0.7 to 0.8. The films combine excellent coefficients of friction with good printability, but are highly unsatisfactory in haze and gloss due to the addition of the friction-reducing polyolefinic resin.
EP-A-0 402 100 describes polypropylene films which contain from 0.01 to 0.5% by weight of a spherical SiO.sub.2 and from 0.3 to 5% by weight of a hydroxy fatty acid glyceride. This invention relates to single-layer and multilayer films. Multilayer embodiments contain the combination of SiO.sub.2 and glyceride both in the outer layer and in the base layer. It is disclosed that the selected amounts of SiO.sub.2 and glyceride are essential for the advantageous properties of the films and deviations from these ranges no longer give the desired result. The films are distinguished by good transparency, surface-slip characteristics and adhesion to metal. However, they develop a coating on the surface after an extended storage time which impairs the appearance of the films. This effect is also known as blooming and is caused by migration of certain additives, in particular the glycerides, to the surface of the films.
EP-A-0 182 463 describes a multilayer film which contains from 0.05 to 0.2% by weight of tertiary aliphatic amine in the base layer and a combination of silicone oil and SiO.sub.2 in the heat-sealable outer layer. According to the description, the surprising interaction of SiO.sub.2, silicone oil and amine in combination with a selected outer layer thickness of less than 0.8 .mu.m gives films having coefficients of friction of 0.3 or less. In spite of this excellent coefficient of friction, the processing properties of the film are poor. In particular, it is not printable and is therefore unsuitable for many applications.
EP-A-0 143 130 discloses films which contain a carboxamide in the base layer and likewise the combination of silicone oil and SiO.sub.2 in the outer layer. Like in the above-mentioned EP-A-0 182 463, a synergistic action of the three selected components on the coefficient of friction is described. These films likewise have poor processing properties in spite of their advantageous surface-slip characteristics. Again, they lack the important property of printability.
EP-A-0 194 588 and EP-A-0 217 598 describe a further-developed film which is readily printable in spite of good surface-slip characteristics, due to the addition of silicone oil. In these films, silicone oil, if desired in combination with SiO.sub.2, is only incorporated into one outer layer. The second, silicone oil-free outer layer is corona treated in order to improve the printability. The transfer of silicone oil to the surface of this treated, second outer layer subsequently takes place owing to contact with the silicone oil-containing first outer layer. This feature gives a silicone oil-containing film having good surface-slip characteristics which is simultaneously readily printable on the corona-treated side and is nevertheless heat-sealable. This film has the disadvantage that it can only be printed on one side. This is particularly disadvantageous for use of the film in the packaging sector. Packaging frequently requires further labeling on its second surface, for example a bar code, and for this reason requires a film having good surface-slip characteristics which can be printed on both sides. In addition, it has proven disadvantageous that the pretreatment intensity of the corona-treated surface is no longer measurable due to the silicone (siloxane). This means that it is not easy for the film processor to check the surface tension of the film, an important quality feature. The processing properties of the film are also unsatisfactory.
By applying the known teaching, it has been found that conventional SiO.sub.2 in the production of the films results in deposits on the die lip and in considerable abrasion on the rolls over which the film passes during production. It has been found that this abrasion is caused by the SiO.sub.2 present in the outer layers. This abrasion results in a number of problems. The die lip and the rolls must be cleaned frequently, since the film otherwise runs poorly during production. The deposits on the die lip result in streaking on the film, which impairs the film's appearance. In addition, these impurities cause problems during corona treatment. The corona treatment breaks through in the areas of the roll where SiO.sub.2 abrasion has occurred and results in the undesired phenomenon known as the reverse-side effect. This causes unacceptable flaws during further processing of the film, such as, for example, printing or metallization. The films finished in the usual way with SiO.sub.2 are also unsatisfactory with respect to their roll make up and their blocking behavior at elevated temperature.